1. Field of the Invention
The present invention relates to an improved small-scale and low-cost optical scanning system for use in a laser beam printer which uses a semiconductor laser as a light source.
2. Description of the Prior Art
An optical scanning system for a laser beam printer consists basically of a light source section which issues a light beam, a deflector for deflecting the light beam, and a scanning lens unit which converges the deflected rays of light at a position proportional to the angle of deflection. A semiconductor laser which is small and directly tunable is commonly employed as the light source. Since light from a semiconductor laser is divergent, it is usually employed together with a collimator lens which collimates the laser beam. The angle of divergence of the laser light emitted from the semiconductor laser differs in two directions, i.e., a direction parallel to the junction plane of the laser (this direction is hereinafter referred to as a parallel direction) and a direction normal to the junction plane of the laser (which is hereunder referred to as a normal direction). Since the angle of divergence is larger in the normal direction than in the parallel direction, the diameter of the parallel beam obtained as a result of passage through the collimator lens is larger in the normal direction than in the parallel direction. As a consequence, the rays of light that are finally converged on a scanning surface by the scanning lens unit have an F number that is smaller in the normal direction, and the spot diameter, which is proportional to the F number of these rays of light, is larger in the parallel direction.
In order to solve this problem, the aperture diameter of the collimator lens has been made sufficiently small so that it blocks unwanted rays of light in the normal direction, thereby producing a beam spot having the same diameter in both parallel and normal directions, at the sacrifice of energy efficiency. An alternative approach has been to employ an anamorphic optical system such as a prism for the purpose of shaping the beam.
Another problem with conventional printers is that a deflector such as a rotating polygonal mirror causes unevenness in the pitch of scanning lines because of "tilting" which involves an error in a direction that is normal to the main scanning direction (which is hereunder referred to as an auxiliary scanning direction).
Two methods have been proposed for correcting this problem. One method consists of placing an anamorphic optical system in front of the deflector so that laser light is imaged on the deflecting plane, which is coincident with a cross section of the scanning optical system taken in the auxiliary scanning direction, with the scanning lens system also being made to have an anamorphic composition that allows laser light to be reimaged on the scanning surface, so that it is conjugative with the deflecting plane to eliminate adverse effects of tilting. The other method comprises using an anamorphic optical system and a scanning lens system in such a way that the focal distance and magnification in the auxiliary scanning direction are sufficiently reduced to minimize the unwanted effects of tilting.
The first method which involves linear imaging of laser light on the, deflecting plane is vulnerable to flaws or dust particles on the deflecting plane. Furthermore, changes in the deflecting point of the rotating polygonal mirror cause such strong effects that it is difficult to maintain desired performance over the full range of the scanning width. The second method necessitates the use of a complex optical system in order to achieve the beam shaping. Furthermore, in order to compensate for the insufficient "tilting" correction, a high degree of dimensional precision is required of the rotating polygonal mirror, leading to increased production costs.
An f.theta. lens characterized by proportionality between the angle of incidence and the height of an image is commonly used in the scanning lens system for converting deflected rays of light on the scanning surface at a position proportional to the angle of deflection. However, in order to attain the proportionality (hereinafter referred to as linearity) between the incident angle and image height, the f.theta. lens has a strong negative distortion and requires that the error that might occur in this linearity be very small.